In many signal processing applications it is desirable to combine two input signals so as to provide the product thereof. For example, it may be necessary in some signal processing systems to form the product of an input radio frequency (RF) signal and a base-band signal. Such signal processing often is required in modem systems, in array antenna systems, in systems using adaptive equalizer circuitry which requires complex multipliers, in interference cancellation systems, or in systems requiring the use of phase shifting circuitry. In such cases it is usually required that a linear multiplication operation be provided over a wide range of input signal levels.
One such circuit which might be considered for use for such purposes is described in U.S. Pat. No. 3,550,041, issued to W. A. Sauter on Dec. 22, 1970. Such patent describes the use of two PIN diodes in a bridge circuit arrangement. While the patent primarily discusses the use of such circuitry for regulating the amplitude of an RF signal with fine resolution over a large dynamic range by utilizing a DC control signal, such circuitry could be adapted for use in forming a product of an RF input signal and the DC control signal.
However, several disadvantages of the circuit disclosed by the Sauter patent arise in this connection. First of all, the circuit requires two bias voltage supplies, one a positive bias supply and the other a negative voltage supply, such bias supply means thereby comprising two independent bias sources for each of the two diodes involved, as disclosed and claimed in the patent. Not only does the requirement for two bias power supplies increase the cost of the overall device, but also the circuit becomes relatively highly sensitive even to small changes in the power supply voltages from either of the bias sources. Such undesired sensitivity arises because the circuit is in a bridge configuration and a change in the power supply level of a bias source affects only the corresponding branch of the bridge, not changing both branches equally.
Another disadvantage is that the Sauter circuit requires the use of substantially exactly matched diodes for its operation over a wide range of control currents. However, no effective technique for adjusting the bias currents to provide such an exact match is suggested by Sauter. If the usual method of matching PIN diodes is utilized, i.e., where a bias current is inserted so that at some value of control current the two diodes have the same resistance, such matching technique does not provide for a match of the diode characteristics over the entire range of control currents which may be involved.
A further disadvantage of the Sauter circuit arises in that the circuit cannot be easily compensated for temperature effects without unbalancing the bridge circuit and thereby adversely affecting its operation.
Hence, the Sauter circuit is not effective in providing linear characteristics for the multiplication of two input signals over a relatively wide range of input signal levels.